Soil structure — The physical aggregation and tilth of soil are especially improved by the accumulation and growth of which component?

Introduction / Context:
Soil structure refers to the arrangement of particles into aggregates. Stable aggregation improves aeration, water infiltration, resistance to erosion, and root penetration. Biological agents, particularly filamentous fungi (molds), play a central role in forming and stabilizing these aggregates.

Given Data / Assumptions:

• We focus on agents that actively improve structure through binding.
• Minerals and water are fundamental constituents but do not inherently create aggregation without organic binders.

Concept / Approach:
Fungal hyphae physically enmesh soil particles and exude sticky biopolymers (e.g., glomalin from arbuscular mycorrhizae), enhancing aggregate stability. While clay–humus interactions and cation bridges matter, the explicit “accumulation of mold mycelium” is a classic factor credited with improving structure and forming stable crumbs in topsoil.

Step-by-Step Solution:

Verification / Alternative check:
Soil science literature documents positive correlations between fungal biomass, glomalin content, and mean weight diameter of aggregates.

Why Other Options Are Wrong:

• Minerals: necessary matrix but not an “accumulation” that improves structure by itself.
• Water: transient; both excess and deficit can degrade structure.
• All of these: overbroad; only the mycelium acts as a primary biological binder.

Common Pitfalls:
Assuming more clay or water automatically improves structure; without organic binding agents, aggregates remain weak.

Final Answer:
mold mycelium